The project that is the subject of this report

was approved by the Governing Board of the National

Research Council, whose members are drawn from the

councils of the National Academy of Sciences, the

National Academy of Engineering, and the Institute of

Medicine. The members of the committee responsible for

the report were chosen for their special competences and

with regard for appropriate balance.

This report has been reviewed by a group other than

the authors according to procedures approved by a Report

Review Committee consisting of members of the National

Academy of Sciences, the National Academy of Engineering,

and the Institute of Medicine.

The National Research Council was established by the

National Academy of Sciences in 1916 to ansociate the

broad community of science and technology Aith the

Academy's purposes of furthering knowledge and of

advising the federal government. The Council operates in

accordance with general policies determined by the

Academy under the authority of its congressional charter

of 1863, which establishes the Academy as a private,

nonprofit, self-governing membership corporation. The

Council has become the principal operating ageacy of both

the National Academy of Sciences and the Naticnal Academy

of Engineering in the conduct of their services to the

government, the public, and the scientific and

engineering communities. It is administered jointly by

both Academies and the Institute of Medicine. The

National Academy of Engineering and the Institute of

Medicine were established in 1964 and 1970, respectively,

under the charter of the National Academy of Sciences.